


Blood's Heat

by SXH1417



Category: Undertale (Video Game)
Genre: Blow Jobs, Breeding Kink, ClassicHorror, Collars, Cum Inflation, Growling, Knotting, M/M, Multiple Orgasms, Rough Sex, Sans has a tail, Size Difference, bara blood - Freeform, cunninglingus, heat - Freeform, slight praise kink
Language: English
Status: Completed
Published: 2018-12-18
Updated: 2018-12-18
Packaged: 2019-09-22 09:26:26
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,922
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/17057156
Author URL: https://archiveofourown.org/users/SXH1417/pseuds/SXH1417
Summary: The very air in Snowdin was tense today. It hadn’t started out that way. It was just seemed to… happen. Everyone was set on edge and no one knew why. A rumor was starting to go around, saying there was a human lurking in the town.Sans quickly figures out that this is not the case.Don't mind me, I'm bad at summaries. This is just smut.





	Blood's Heat

**Author's Note:**

  * For [yastaghr](https://archiveofourown.org/users/yastaghr/gifts).



> Thank you to the lovely person who requested this! I hope you enjoy!

The very air in Snowdin was tense today. It hadn’t started out that way. It was just seemed to… happen. Everyone was set on edge and no one knew why. A rumor was starting to go around, saying there was a human lurking in the town. The dogs set about patrolling the area, making sure everyone was in their homes to keep them safe.

Sans didn’t know about any of it. He had just gotten off his shift and teleported to Grillby’s. He knew something was off when he wasn’t greeted with the usual cheer. He wasn’t greeted at all. Grillby seemed to be closing for the day, putting away all the alcohol and cleaning tables. When Sans asked him what was going on, Grillby told him about the curfew, how all the dog guard had gone to patrol. Sans asked what was wrong and Grillby shook his head. He explained that no one knew and that he should get home too.

Worried, Sans teleported to his house and knew exactly what was going on. He could feel the dangerous air even from outside. It made his tail tuck between his legs and curl tightly around his leg and his fingers grip tightly to the thick black collar around his neck. He whimpered, feeling predatory eyes on him. He stepped inside and found Papyrus and Sugar huddled in the kitchen. Papyrus was trying to comfort Sugar, who looked a bit shaken.

“where is he?” Sans asked.

“Upstairs,” Papyrus replied simply, pulling Sugar into an embrace when he whimpered a little.

“what happened?” Sans said as he came to check Sugar too. Sugar looked okay, physically.

“Blood was sitting down with us, watching MTT when all of a sudden he shoved us aside and stormed upstairs. There have been… noises… since then.”

Sans’s brow furrowed. “What kind of noises?” As soon as he asked there was a loud crash, followed by growling. It sent a shiver up Sans’s spine.

“Those noises…” Papyrus murmured. Suddenly, he took a small sniff of the air and cringed. “I-I’m going to take Sugar out for a slumber party at Undyne’s! How does that sound Sugar?”

“O-okay…”

“GREAT! I’M GOING TO GO BACK. NOW YOU JUST WAIT OUTSIDE! I’LL BEBACKLICKETYSPLIT!” Papyrus stammered as he all but shoved the other outside. He promptly ran upstairs while Sans dialed Grillby’s.

 _“What now, Sans?”_ Grillby’s voice crackled on the other side.

“Grilbz, everythin’s alright. my friend is just in a _very_ dominant heat right now. please tell me you didn’t clean the grill yet,” Sans stammered.

_“Wha – Sans, what on earth are you saying? One monster is causing all this unease?”_

“yep. he’s uh… an uneasy sort of guy. but, that aside, i seriously need to know if you closed down the grill.”

_“No, not yet, but I don’t see how that’s –“_

“good, i’m gonna to need about three or four days-worth of your burgs and fries stat. charge it to my tab and i promise to pay it off as soon as i see you again. Once they’re done just set them on the inside of the door to my house. key is under the mat.”

_“Sa-“_

Sans hung up before he could hear the rest. He was more preoccupied with helping Blood right now. Papyrus closed his door quietly and slipped down the stairs with two suitcases. Papyrus, seeing Sans heading up the stairs, stopped him. He swept him up into a tight hug.

“Be careful, brother… If he hurts you, I don’t think it will be on purpose. He’s been like this for a while,” Papyrus said.

“don’ worry about me, bro. i’ll be fine. besides… you know he won’t hurt me…” Sans’s fingers brushed against the collar. Papyrus didn’t respond, skepticism written on his features. He gave Sans one last hug before heading toward the door. When Sans heard the tell-tale click, he tensed. The noise sounded final like it was sealing his fate.

Steeling himself, Sans took a deep breath before he started heading upstairs. The higher he got, the more he could smell what Papyrus smelled. It was deep and musky and made Sans’s head spin a little. He clutched at the collar again. Feeling the smooth leather calmed him a little. Blood wouldn’t hurt him on purpose.

Still… Sans felt his patellas click together a little as he stared down the hall toward his room. That deep musky smell always had a sharpness to it. A metallic like smell that reminded Sans too much of Blood’s namesake. Nonetheless, it had his body reacting. His magic was already gathering as the air around him covered him, almost gently coaxing him to submit, to let go.

Sans shook his head a little to clear it. He had to get to Blood and help him. He slowly inched forward, putting one foot after the other until he was at his own door. He heard the rustling sounds and growling on the other side of it. It sent little shivers up his back with thoughts of those same sounds being made against his neck.

Reaching for the handle, he hesitated. This wasn’t the first time he shared a heat with Blood, but he still hesitated. Blood’s heats had always tended to be on the more… feral side. Sans had scars from the first time he mated with Blood. The collar only hit part of those teeth marks. On top of that, the intensity always varied. So… he kind of lied to Papyrus. He had no idea if Blood was going to hurt him. He hoped not. He hoped that the collar that Blood gave him would discourage any real harm.

He took another deep breath. He could do this. This was his mate and his mate needed help. He wrapped his handle and tried to open the door. It was locked. Of course. Blood locked himself inside as soon as he went into heat. It kept him from hurting Papyrus or worse… Sugar.

Taking this opportunity, Sans gently knocked on the door. All sounds stopped from within the room, making Sans tense more. He could hear Blood’s low breathing. He could feel Blood’s eyes on him through the door.

“h-hey… Blood?” Sans called. He didn’t get an answer. “a-are you alright in there?” Okay. That was a dumb question. Blood wasn’t okay. He was experiencing one of the worst heats in his life. “i-i’m here to h-help… c-can you unlock the d-d-door? P-please?” He heard a loud whine come from inside the room. Okay. Yeah. Blood _wasn’t_ okay. He was too far gone, but still had enough control to stop his own movements. Sans would have to unlock it himself. “h-hey. it’s alright. i’m c-coming in. i’ll be able to help ya in a minute, just stay there.”

Sans fished out the key to his room. As soon as he found it, he slowly opened the door. The room was dark, but Sans could see Blood’s large, blown eyelight illuminating the room. Shit. The room itself was in tatters. The treadmill was toppled over and against the wall, the trash tornado had bits of fabric flying in it, and his bed was shredded.

And the smell. It was twenty times stronger in here and it felt like a punch to Sans’s ribcage. He more than ever just wanted to kneel down and present himself in front of Blood, but that would likely be the feather to break the camel’s back and all.

Blood had half his shirt stuffed between his teeth, muffling any sound that came out. He was hunched over the bed, hands gripping onto Sans’s pillow that was wedged between his thighs. Blood’s cock could be seen clear as day resting on the pillow, moving here and there due to blood’s twitching hips. He looked at Sans both pleadingly and demandingly. His single eyelight was fuzzy, almost like static with how hazy they were. He was almost _too_ far gone.

Sans had to help him quick. Biting back his instincts, he slowly hobbled closer to Blood. Stars, his size always intimidated Sans. He was a good two or three heads taller than Papyrus, making him a little over twice Sans’s size. When Sans touched the bed with his hands, Blood let out a low rumble. The smaller skeleton shivered, his magic snapping into place and taking the form of a pretty, blue vulva. His tail tightened around him, putting sweet pressure on Sans’s clit.

Blood’s eye widened further at the sighed, hands twitching to snatch him up and use him to sate his needs. Sans nearly moaned from the thought. A small fantasy already playing out in his head of Blood using him, forcing him to take all of him, and then filling him up so much that his stomach distended, so much that blood would breed him. He closed his newly formed thighs as liquid began to seep from his conjured opening.

Sans shook his head, trying his best to clear out some of the fog of heat that had taken hold. He slowly climbed onto the bed, noting that Blood was stiff as stone. He was shaking from trying to hold himself still.

“y-yer doing so great, B-Blood,” Sans began, scooting closer. “you h-haven’t lost control y-yet. you are doing so well…” Sans slowly pressed himself closer. If he could get Blood off once before he lost complete control, then the heat would die down to something more manageable. Something that wouldn’t hurt. Blood’s hips jerked a little at his praise. “don’ worry… i-i’m here to help ya now… j-just k-keep still…” Sans said as he sunk lower.

The smaller skeleton had to pry the pillow out of Blood’s grip. He made a note to wash it later with how soaked in precum it was. He gently pushed Blood down into a sitting position. Once there, Blood would go no further. His body locked up, refusing to take a ‘submissive’ position. Sans understood this and got in between Blood’s femurs.

Blood’s cock was dark, almost maroon. It was clear he hadn’t cum once since his heat started. The pillow was just not enough. On top of that, his heat made him form a knot. Sans sighed. With how pent up Blood was at this point, he would probably need to get off two or three times before sex would be safe enough to attempt. Sans ran his hands over Blood’s exposed femurs, a gesture that was supposed to be comforting but only seemed to rile Blood up. Blood placed his hands over Sans’s, keeping him still.

Sans realized that was about as much touching he was going to manage through this. He leaned down to take another look at Blood’s cock. It was steadily oozing pre. Sans took it slow as he leaned down. He pressed his teeth against it. Blood jolted but otherwise remained still. He watched and waited for Sans to do something else.

Sans went back to work, pressing his teeth to Blood’s cock here and there, letting him get used to the feeling again. He pressed a skeleton kiss to the tip and the cock throbbed against his teeth, pumping out more pre. Sans met Blood’s eyes, noting the flush on his face that steadily grew brighter. His grip tightened on Sans’s fingers as Blood’s eye zeroed in on that blue tongue peeking out of Sans’s mouth. Sans wasted no time in licking a long line up Blood’s length.

“Hrk… hnnn!” Blood jolted again, stopping his hips from twitching into the sensation. He began panting harshly like he suddenly remembered to breathe. His sockets were half-lidded. He let out a small yelp when Sans’s tongue touched him again. This time, Sans slowly took in the head of Blood’s cock into his mouth. Sans could tell he was close. He resumed his fingers ministrations on Blood’s femurs. “mmmmph… mmmm… mmmaa… haaahhh… s… sansss….” Blood breathed, letting his shirt fall from his mouth.

Sans barely paid his words any mind. He was slowly working himself to try and take in more of Blood. His jaw could only take so much before it popped out of place. As he took more of it, Blood’s cries grew. His hips twitched a little, forcing Sans to take more in. Sans pulled back, starting to bob his head on Blood’s cock. He would lick a broad swipe across the head, teasing the slit before taking more of the length in.

Blood didn’t last much longer than that. He tensed up, shredding more of his bed under his claws as he came inside Sans’s mouth. Sans swallowed as much as he could, knowing what it would do to Blood. The larger monster let out a loud, rumbling growl as Sans slowly pulled off.  Some of the cum slipped down his mandible as he swallowed the last of it.

“thank you, precious…” Blood muttered, still panting. “now get up here.” The words trailed off into a snarl, warranting no argument or protest. Sans was snatched up with a small squeak which was quickly swallowed by Blood in a bruising kiss. Blood set to work relieving Sans of his clothes as he continued to claim his mouth. Sans whimpered as Blood’s fingers scrapped his ribs, rubbed his spine and gently unwound his tail from around his hip. “come on, precious, show me…” Blood said as he moved Sans’s tail. Blue slick clung to the underside, a small line connecting it to Sans’s soaked shorts. “yer really affected by this aren’t ya…”

Sans could only muster a small nod as he held onto Bloods shoulders while Blood divested him of his shorts. With nothing to contain it, Sans’s slick began to trickle down his thighs and over the tail as it rewrapped itself around one. Blood gave a loud growl of approval.

Pushing Sans back on the plush nest, Blood got between his legs. He could smell Sans’s anxiety as he tried to line them up. Deciding to comfort his mate, Blood fought down the raging heat inside him and lowered his head between Sans’s legs. Being so close to Sans was like being next to ice on a hot day. It was refreshing. Sans’s magic staved off the heat so well that Blood’s tongue licked out to touch the wonderfully cool air. A little taste wouldn’t hurt…

Sans nearly screamed when Blood’s tongue licked against his labia and up to his clit. It was a satisfying pleasure that he wanted more of. Blood was more than happy to give as he continued to lick at Sans’s lips, dipping inside to taste the magic that dripped out in such copious amounts. His hands shot to Blood’s skull, scrabbling to get a hold of something, anything to ground himself. One hand clung desperately to the edges of the hole, causing Blood to rumble. Sans couldn’t think whether or not it was a good rumble or a bad one. His sole focus was on that tongue working its way deeper inside of him.

Blood’s hands grasped Sans’s thighs and pushed them higher, allowing him to dig his tongue deeper inside of Sans. Sans was a moaning mess as Blood took his time to really taste him. He would dip his tongue inside, thrust it in and out and pull away to flick it against Sans’s cute little clit. Stars, he wanted to hurry up and take Sans, but he didn’t want to hurt him. While he worked, he reached down to tug on his cock, trying to obtain some relief while he waited.

Just imagining being inside of Sans had Blood close. He couldn’t wait to fill him up, cum inside him over and over until he was choking on his cum. He wanted to pin Sans down, knot him, and bite right through his collar. He’d be disappointed at the waste, but he would buy Sans a new collar, a red one, just like his magic. Blood’s jerked himself faster, making sure to give Sans the same treatment. He sucked at his labia, his clit, pressed a few fingers inside to stretch his little mate.

“Ahh! AAaAH!” Sans screamed as he came around Blood’s fingers. His thighs clung to both sides of his head as his fingers dug into the hole. Blood was a little shocked that Sans came so quickly. His surprise was covered with a growl as his heat resurged. Sans’s magic had cooled him only temporarily. He needed more…

Flipping Sans over onto his front, he lined them up. He gave Sans a few kisses, grinding his erection into those soaked lips as a warning before he pushed slowly pushed inside. Sans flailed under him, crying out a little in pain. He knew it hurt. It hurt Blood to hurt him like this, but it wouldn’t hurt for long. Grabbing a hold of Sans’s hips, he continued to enter Sans until he his knot pressed against Sans. Blood felt a little dizzy from all the pleasure that hit him. He leaned over Sans, holding himself up with one hand while he tried to adjust to the feeling, trying not to come again so quickly.

Sans’s eyes were brimming with tears. The sensitivity from his orgasm both heightened and lessened the pain, leaving him with the intense burning sensation as Blood’s cock forced it’s way deeper and deeper inside of him until he could swear that he felt it touch his lumbar vertebrae. One thing was for sure. The heat that Blood produced was searing, heating him to his very core in such a comforting way that he wanted more. When the knot touched him, he shuddered. Sans didn’t want to imagine how it would feel getting that thing inside of him. Although, his mind was supplying wonderful images of how it would feel, both pleasurable and painful. He must be one hell of a masochist to be looking forward to both…

Blood stayed still for several minutes, panting above him. Luckily it was more than enough time for Sans to adjust to the stretch. The searing heat that his own body produced, a byproduct of Blood’s heat, a bleeding affect, made him want more. Sans rocked back into Blood, enjoying what little friction it caused before Blood’s hand tightened and stopped his movements.

He whined loudly, pleading, “please… please, please, please move… Blood, please…”

Blood growled against his neck, giving one punishing thrust that had Sans keening. He didn’t try and move further, but his pleas didn’t stop. It fueled a primal part of Blood to hear his mate begging to be fucked. He bit into the leather of Sans’s collar to help calm himself down.

Once he got to where he could move without coming immediately, he did. His heat wouldn’t let him go slow once they set a good pace. It wanted him to stuff his cock into his mate faster and harder. He thrust into Sans’s pliant body, hitting that one spot Sans loved more than anything. It filled Blood with a great deal of satisfaction to hear his mate screaming from him, to hear how eager he was.

Blood continued his fast pace, plowing into Sans so hard that his formed ass rippled a little. He was so cute and adorably sexy… Blood fondled Sans’s cheeks, looking down to where his knot caught inside of Sans and moaned. Sans’s slick was everywhere, coating his knot and his thighs.

“yer takin’ me so well, precious… so well… fuck, yer soaked… mine… yer mine. only i can do this to you. my sans… _mine_ ,” Blood snarled as he thrust harder.

Sans felt a like a cocksleeve. He was being used so thoroughly and he loved it. Blood’s fingers were digging painfully into his hips now, likely to leave bruises or even cracks in his bones. Sans wanted to see them later. He wanted to see the marks and know that Blood claimed him again. He would love to show it off to everyone so they know he was so thoroughly owned…

Blood pulled out suddenly and flipped Sans over. Sans cried only in protest only for it to end in a moan as Blood thrust inside him again. Stars, he could see Blood’s glowing eyelight so brightly. It was staring down at him with so much love and lust. Blood could see everything he was doing to him and Sans couldn’t do anything but cling to him. His tail wrapped tightly around Blood’s finger, twitching.

Blood reached down and tugged the collar down to expose the side of his mate’s neck. He bit down on it. Hard. He drew marrow and it only spurred him on. Stars, why did Sans’s magic feel so perfect around his own. Why was this pristine little guy so perfect… So precious… Reaching down he snatched Sans’s tail from around his femur and stroked it. Sans body spasmed when he played with the tip.

Sans squirmed a little more before he came around him, squeezing his cock tightly. That was all it took for Blood. He thrust in several more times before forcing his knot inside. The scream Sans let out was one of both pain and pleasure. He felt Sans come again once it was inside. Blood let out a choked moan, lightly rocking Sans as he started pumping him full.

“so precious… so perfect… gonna be a good momma… my little momma… give me so many babybones… stars…” Blood cooed, holding Sans close, running his hand over his ribs lazily and over the steadily swelling stomach. “yer so amazing… i love you, Sans… love you so much…” The rocking became a steady motion, making Sans cum and then making Blood cum again. Blood didn’t want to stop the rocking motion. He wanted to keep this. He needed to keep this closeness with his mate. His phalanges grazed the collar, the texture sending pleasurable little shocks as he came again. Sans was his…

 

* * *

 

Sans was on the verge of passing out when Blood’s knot finally released them. He clung weakly to Blood was he was laid down on the nest to rest. Blood pulled away and went somewhere, likely to hunt down some food for his mate. Sans took a moment to breathe. Blood’s heats were always so intense...

Sans was on the brink of unconsciousness when Blood shook him awake. Sans looked up at him blearily before noting that there was a burger in Blood’s hands. Blood had that adorable look on his face again. He was always happy to feed Sans after this. He looked so proud to have ‘found’ food for his mate. Sans smiled, nibbling on the food as much as he could. Blood wouldn’t let him rest until he at least finished off some fries.

After he ate, he tried to get comfortable to rest. His body was screaming at him with how sore his was. he squirmed a little, trying to get into a comfortable position while Blood’s cum oozed out of him. Blood laid beside him and pulled him close to his chest. His head rested on Sans’s as he began to purr in contentment. The slow rumble began to make Sans doze.

He slowly dipped off into sleep, knowing that when he woke, he was going to be put through much of the same treatment. With how determined Blood was, he likely would have one or two babybones in him by the time this was all over with. Sans didn’t mind. It would just show everyone who be belonged to even more.

**Author's Note:**

> I hope you enjoyed the story! Please let me know what you think with a comment!
> 
> Also, I am no longer on Tumblr.  
> My Twitter is @sxh1417. Do not follow me if you are below the age of 18. It is required that you have your BirthYear public if you follow me. Otherwise, you will be blocked with no hope of ever being unblocked.


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